Seagrass meadows, seabed sediments, mussel reefs and macroalgal forests can take up and retain carbon. But how much of that carbon remains sequestered over the long term? This scientific literature review examines carbon retention and sequestration across four marine habitats in the North Sea and Baltic Sea. It identifies the processes that determine whether carbon is stored permanently, highlights substantial knowledge gaps and explores the implications for nature-based climate mitigation.

Carbon uptake does not necessarily mean long-term sequestration

Marine ecosystems play an important role in the global carbon cycle. Seagrasses and macroalgae take up carbon dioxide through photosynthesis, while organic matter can accumulate in seabed sediments. Whether this carbon remains sequestered over the long term or is returned to the carbon cycle through biological and physical processes depends strongly on local environmental conditions.

The scientific literature review 'Retentive structures of carbon in the North and Baltic Seas' examines these processes across four benthic habitats: seagrass meadows, subtidal sediments – seabed sediments that remain permanently submerged – biogenic reefs formed by mussels and oysters, and macroalgal forests.

The review demonstrates that carbon sequestration cannot be assessed solely on the basis of the amount of carbon taken up or retained. Equally important are the stability of carbon stores, the transport of organic matter and the processes through which retained carbon is remobilised and returned to the carbon cycle.

Four habitats with distinct carbon retention functions

Subtidal sediments account for the largest share of regional carbon stocks because of their extensive spatial coverage. However, a substantial proportion of deposited organic matter is remineralised or resuspended by currents and wave action. Deep, relatively low-energy environments, such as the Norwegian Trough, provide particularly favourable conditions for long-term carbon burial and sequestration.

Seagrass meadows, by contrast, are highly efficient at retaining carbon per unit area. Their leaves reduce water flow and promote the deposition of suspended organic particles, while their below-ground structures stabilise sediments and help protect stored carbon from resuspension. However, their relatively limited spatial extent means that their contribution to overall regional carbon budgets remains small.

Biogenic mussel reefs also influence carbon cycling. Through filter feeding, mussels remove organic particles from the water column and promote their deposition on the seabed, including in the form of faeces and pseudofaeces. At the same time, shell calcification can lead to the release of carbon dioxide. Whether mussel reefs act as net carbon sinks or sources therefore remains a subject of scientific debate.

Macroalgal forests take up substantial amounts of carbon during growth. Some of this carbon is exported as detached algal biomass to deeper marine environments, where it may contribute to long-term sequestration. Of particular interest is fucoidan, a relatively resistant organic compound released by brown algae that can persist in the marine environment. However, the amount of carbon permanently sequestered through these pathways remains insufficiently understood.

Human pressures threaten marine carbon retention

The review shows that human activities affect carbon retention and sequestration in the North Sea and Baltic Sea in different ways. Eutrophication, bottom trawling, pollution, overexploitation and climate-related changes have contributed to the degradation and loss of important marine habitats. These impacts are particularly evident in seagrass meadows and macroalgal forests.

Excessive nutrient inputs can stimulate phytoplankton growth, reduce light availability and thereby impair the growth of benthic vegetation. Reductions in nutrient loading have already contributed to the recovery of seagrass populations in some regions.

Physical disturbance of the seabed can also alter carbon retention. Bottom trawling, for example, resuspends sediments and may enhance the remineralisation of organic carbon. However, the magnitude of this effect varies between regions and studies.

Climate change introduces further uncertainties. Rising water temperatures may accelerate the microbial decomposition of organic matter, while marine heatwaves and changing hydrodynamic conditions place additional pressure on sensitive habitats.

Better evidence for nature-based climate mitigation

A central conclusion of the review is that the available evidence remains insufficient to establish robust regional carbon budgets. Existing studies differ considerably in their methodologies, spatial coverage and the carbon cycling processes they account for.

A further challenge arises from the transport of carbon between habitats. Organic matter originating in seagrass meadows or macroalgal forests may subsequently be deposited in deeper sediments. If the carbon retained by these habitats and the carbon buried in receiving sediments are counted independently, the same carbon may be included more than once in regional estimates.

The authors therefore emphasise the need for harmonised monitoring approaches, spatially resolved assessments and improved models. Emissions of other greenhouse gases, including methane and nitrous oxide, must also be considered to assess the overall climate effects of marine habitats.

For nature-based climate mitigation, the findings have important implications: protecting and restoring healthy marine ecosystems can make a valuable contribution. However, a better understanding of the underlying carbon cycling processes is essential to quantify their climate mitigation potential reliably and to inform the design of targeted measures.

The scientific literature review was produced as part of the research project 'Strengthening the Carbon Storage Potential of the North Sea and Baltic Sea'. The project investigates the carbon storage potential of the North Sea and Baltic Sea under good ecological and environmental conditions.